Using intracellular microelectrode techniques, we are studying the effects of changes in the mucosal bathing medium on the electrical pathways in toad urinary bladder. Membrane potentials are directly measured, and, after determining the resistances of both cell borders and of the paracellular shunt path, it is possible to compute the electromotive force (the PD which would exist in a circuit where it is the only element present) at each border. With this technique we are studying the permselectivity of the outer membrane to both cations and anions. Furthermore, we are currently injecting sodium into the cells and studying its effects on the measured PD's and resistance; this method should help determine whether or not the sodium pump mechanism is itself capable of transferring charge (electrogenic or rheogenic). In other studies we are measuring shunt resistance by determining the serosa-to-mucosa fluxes of Na, K, or Cl. We have shown that these ions traverse the same path in this direction and this path is clearly extracellular. We can thus determine abrupt changes in shunt resistance by measuring fluxes at 6-30 second intervals following sudden imposition of osmolality changes in the mucosal solutions. By doing these studies under differing conditions of transport activity, we have been able to establish a relationship between the sodium transport system and the conductance of the paracellular pathway.